1. Technical Field
Methods for forming wall oxide films and isolation films for flash memory devices are disclosed. More specifically, a disclosed method for forming wall oxide films prevents a “dislocation” phenomenon where a sidewall within the trench is broken due to thermal stress caused by an oxidization process performed after the trench is formed. A method of forming an isolation film in the flash memory device using the same is also disclosed.
2. Description of the Related Art
Recently, there is an increased demand for flash memory devices which can be electrically programmed and erased and which do not need a refresh function that data is rewritten in a given period. “Program” refers to an operation for writing data into memory cells, and erase refers to an operation for erasing data written into memory cells. Furthermore, research into higher-integration technology of memory devices has been actively conducted in order to develop memory devices with large capacities.
In a flash memory device, an isolation film is formed by means of a shallow trench isolation (STI) process in order to electrically isolate neighboring elements (for example, cells and transistors). Typically, a STI process is performed by forming a pad oxide film and a pad nitride film on a substrate, performing a trench etch process to form trenches in the substrate, and then depositing an insulating film so that the trenches are buried or filled, thus forming an isolation film.
The STI process includes a wall oxidization process, which is performed on the sidewall within the trenches in order to compensate for damage to the sidewalls of the trenches, which are damaged by the trench etch process or to control the area of the active region, after the trench etch process is performed. Wall oxide films are formed on the sidewalls of the trenches by this wall oxidization process.
Generally, the wall oxidization process is carried out in a furnace, a high thermal stress is caused, and a dislocation phenomenon in which the silicon sidewall portion is broken at the top/bottom regions of the trenches is generated. This dislocation phenomenon generates a path along which the leakage current flows, and thus causes device characteristics to degrade. Furthermore, in an ion implant process for forming subsequent source and drain regions, the dislocation phenomenon causes defects to occur in an active region as well as the source and drain regions.